Films formed from fluorine-containing polymers have good properties such as heat resistance, abrasion resistance, non-tackiness, low frictional property, chemical resistance, electrical insulation, etc. In particular, heat-meltable fluorine-containing polymers are used in the form of a powder coating in various applications, for example, cookwares such as rice cookers, fry pans, etc., rolls of office automation equipments, pipes and tanks of chemical plants, and the like.
However, the chain terminals of the fluorine-containing polymers may have a small amount of —COF groups due to the polymerization mechanisms. In the case of emulsion polymerization using a polymerization initiator such as ammonium persulfate, —COOH groups are formed at the chain terminals of the polymer. When methanol is used as a molecular-weight adjuster (a chain transfer agent), terminal groups such as —COOH, —COOCH3, —CH2OH, etc. are formed. Since these terminal groups are thermally unstable, they may cause bubbles or generate hydrogen fluoride during the processing of the polymers, and also deteriorate the excellent properties of the films of the fluorine-containing polymers. Thus, the unstable terminal groups are converted to —CF3 group by contacting a fluorinating agent such as fluorine gap with the fluorine-containing polymers (JP-B-08-30097).
A powder coating is often applied to an article by an electrostatic coating method. In particular, in the electrostatic coating method using a corona-charging system, which is most commonly employed, a high negative voltage is applied to the tip end of a spray gun so that an air surrounding the tip end are negatively ionized. Then, powder particles are supplied from a powder-supplying equipment with an air to the spray gun and negatively charged when they pass through the tip end of the spray gun. Thereafter, the charged powder particles are adhered to an article which is grounded by electrostatic attractive force. In this step, the fluorine-containing polymer, most of the unstable terminal groups of which are converted to —CF3 groups, is not easily negatively charged so that a coating efficiency of the powder is very low.